(a) Field of the Invention
This invention generally relates to a device or system for absorbing energy or decelerating an object, such as a falling person. More particularly, but not by way of limitation, to a strap having a connected area designed to tear by breaking the connective threads to create mechanical hysterisis under load and thereby absorb energy.
(b) Discussion of Known Art
The use of sewn sections of strap in order to provide a shock absorbing or energy absorbing member for lanyards is well known. See for example U.S. Pat. No. 3,444,957 to Ervin, Jr., incorporated herein by reference. A significant drawback to these devices is that they are often misused by individuals in the field. A common manner in which these devices are misused is by using a device that has been designed for a moderate fall in all situations. For example, a popular version of these shock absorbing lanyards are six-feet in overall length and are designed for providing an arrest force of 900 pounds of rip out or arrest force as the shock absorber separates during fall arrest. However, these shock absorbers are often used in situations where a fall that is longer than six feet is possible, such as where a person must stand or climb above the lanyard anchor point, or where the attachment to an anchorage point at their feet. Therefore this application requires a shock absorber that provides a high arrest force. The problem associated with using a shock absorber that provides too small of an arrest force for an extended fall (in this example, greater than six feet) is that the fall will cause the shock absorber to extend fully, until it has reached its full energy capacity and is fully extended. Once the lanyard is full length, with the shock absorber fully extended, the person attached to the lanyard is brought to an instantaneous, complete, stop. Such an abrupt stop can expose the worker to force spikes above acceptable levels and result in severe injury to the person relying on the lanyard for safety.
The U.S. Occupational Safety and Health Administration (OSHA) requires that fall protections systems, such as lanyards with shock absorbers, must be rigged and designed such that a falling worker will not fall more than six feet before beginning the deceleration process. Additionally, the deceleration process may not expose the falling worker to arrest forces greater than 1800 pounds, and the deceleration process may not take longer than 42 inches to decelerate the fall to rest.
Today's manufacturers produce two main types of shock absorbers. The first type, known as a Force level I shock absorber, is designed to arrest a six foot fall of a 220 pound person in 42 inches. The Force level I shock absorber provides a maximum arrest force of 900 pounds as it is torn out.
In many applications, there may not be a convenient overhead location where the six-foot long lanyard can be anchored to. For example, a six-foot tall ironworker may only have the steel beam at his feet from which to connect the lanyard. This will result in the exposure of the worker to a 12-foot free fall (the height of the worker plus the length of the lanyard, which is anchored near his feet). In this situation the use of the six-foot lanyard with the Force level I shock absorber is inadequate. As stated above, the Force level I shock absorber is designed to protect the worker in a six-foot fall, but in this example the worker has inadvertently anchored the lanyard in a manner that will result in a 12-foot fall. Accordingly, the Force level I shock absorber is not capable of absorbing all of the energy-gathered during this fall, and will use its entire capacity and suddenly stop stretching when the tear away stitch reaches it's end. This sudden stop will result in a massive jerk to the falling worker. As can be anticipated, a massive jerk is highly likely to injure the falling worker.
Instead of the 900 lb Force level I shock absorber, a shock absorber with a higher energy capacity is required in order to stop the longer fall within the required 42 inches. The next larger available shock absorber is a Force level II shock absorber, which provides a maximum arrest force of 1800 lbs, instead of the 900 lbs provided by the Force level I shock absorber. Thus, at first glance it appears that the solution to the problem of using too small of a shock absorber is to simply provide a higher arrest force shock absorber.
Unfortunately, the use of a Force level II shock absorber, with its 1800 lbs of deceleration force has significant disadvantages. One of the most significant disadvantages is that the 1800 lb deceleration force may itself injure the worker because it will create a greater than 10 G deceleration load on any worker that weighs less than 180 lbs. Arrest force greater than 10 g are not allowed by regulation.
The combined lanyard/shock absorber combination is typically provided together. The reason for providing these two items together is to ensure that the shock absorber is always matched to a lanyard that does not exceed six feet in length. However, as explained above, the length of lanyard does not guarantee that the worker will not be exposed to a larger than anticipated fall. Accordingly, there remains a need for a single fall arrest shock absorbing lanyard that may be used with short freefall distances (less than six feet) without exposing workers to large forces, and provide an added safety margin for situations where the worker anchors the lanyard to a location that will expose the worker to falls greater than six feet.